1. Field of the Invention
The present invention relates to a deflecting apparatus for a color cathode ray tube and a color cathode ray tube apparatus, and more particularly, to a deflecting apparatus for a color cathode ray tube which has a saturable reactor for changing a horizontal deflection current flowing through a horizontal deflection coil that generates a magnetic field deflected in a direction parallel to a direction along which electron beams are aligned, in synchronism with a vertical deflection current flowing through a vertical deflection coil that generates a magnetic field deflected in a direction perpendicular to the electron beam aligning direction, and a color cathode ray tube apparatus.
2. Description of the Related Art
Generally, a color cathode ray tube apparatus shown in FIG. 1 has an envelope having a panel 1 and a funnel 2 that are integrally bonded to each other. A shadow mask 3 having a large number of apertures formed therein to pass electron beams therethrough is mounted on the inner side of the panel 1, and a phosphor screen 4 having a three color phosphor layers that emit blue light, green light, and red light is formed on the inner surface of the panel 1 to oppose the shadow mask 3. In this color cathode ray tube apparatus, three electron beams BR, BG, and BB emitted from an electron gun assembly 6 disposed in a neck 5 of the funnel 2 are deflected in the horizontal and vertical directions by a magnetic field generated by a deflection yoke 7 mounted on the outer side of the funnel 2 to scan the phosphor screen 4. As the result of this electron beam defection scanning, a color image is displayed on the phosphor screen 4.
The deflection yoke 7 for deflecting the electron beams BR, BG, and BB is usually constituted by a pair of saddle type horizontal deflection coils 8 through which a horizontal deflection current flows to scan the electron beams in the horizontal direction, a pair of vertical deflection coils 9 through which a vertical deflection current flows to scan the electron beams in the vertical direction, and separators 10 between the horizontal and vertical deflection coils 8 and 9, as shown in FIG. 2. I the deflecting apparatus shown in FIG. 2, the vertical deflection coils 9 comprise a pair of upper and lower toroidal deflection coils. The vertical deflection coils 9 may comprise a pair of right and left saddle type deflection coils, as is known well.
In color cathode ray tube apparatuses of this type, an in-line type color cathode ray tube apparatus is widely used. In an in-line type color cathode ray tube apparatus, an in-line type electron gun assembly is incorporated, in which three electron guns are horizontally aligned in line to emit three electron beams consisting of a center beam and a pair of side beams. The in-line type color cathode ray tube apparatus employs a self convergence system to form an non-uniform magnetic field, in which horizontal and vertical deflection magnetic fields generated by the deflection yoke have a pin cushion shape and a barrel shape, respectively. The three electron beams are self-focused on the phosphor screen by this non-uniform magnetic field.
In this self convergence and in-line type color cathode ray tube apparatus, however, various types of screen distortions occur due to the characteristics of the tube and the tube assembly error. One of such screen distortions is a cross convergence error, in which deflection of the electron beams in the horizontal direction and a convergence error occur simultaneously. Due to the convergence error, cross convergence error patterns as shown in FIGS. 3A to 3D are displayed on the screen. Regarding correction of such a cross convergence error pattern, conventionally, Published Unexamined Japanese Patent Application Nos. 57-206184, 2-194791, and the like disclose a color cathode ray tube apparatus comprising a saturable reactor for differentially changing the current flowing through a pair of horizontal deflection coils in synchronism with a vertical deflection current to change the shape of the horizontal deflection magnetic field on a time-base manner, thereby correcting the convergence error.
Usually, the saturable reactor consists of a first impedance control coil connected to an upper one of a pair of upper and lower horizontal deflection coils and wound on a saturable core, a second impedance control coil connected to the lower horizontal deflection coil and wound on another saturable core, and saturation control coils connected to the vertical deflection coils.
The direction of a magnetic field generated by the saturation control coil similarly wound on the saturable core on which one impedance control coil is wound is opposite to that of the magnetic field generated by one impedance control coil, and static magnetic fields are applied to these impedance control coils in advance.
The function of the saturable reactor will be described with reference to FIGS. 4A and 4C. Referring to FIG. 4A, an axis of abscissa H represents the strength of the magnetic field generated outside the saturable core, and an axis of ordinate L represents the inductance of the impedance control coils. Referring to FIG. 4A, solid and broken lines 12 and 13 represent the L-H characteristics of the two impedance control coils. Reference symbol H.sub.mag indicates a static magnetic field applied from the outside of the saturable cores; and H.sub.vm, a magnetic field generated by the saturation control coils. A curve 12 indicated by the solid line and a curve 13 indicated by the broken line are symmetric with each other about the static magnetic field H.sub.mag, because the magnetic fields generated by the saturation control coils and applied to the two impedance control coils are directed in opposite directions. When a vertical deflection current flows in the saturation control coils, the magnetic field H.sub.vm is generated, and the static magnetic field H.sub.mag and the magnetic field H.sub.vm are added so that inductances L.sub.u and L.sub.d of the impedance control coils are changed in synchronism with vertical deflection. FIG. 4B shows changes in inductances L.sub.u and L.sub.d. Referring to FIG. 4B, the axis of ordinate represents the inductance, and the axis of abscissa represents a vertical deflection current. The correction amount for a cross convergence error by such a saturable reactor is almost proportional to the difference between the inductances L.sub.u and L.sub.d of the two impedance control coils. Thus, the correction amount plots the correction pattern indicated by a curve 16 shown in FIG. 4C.
Conventionally, a cross convergence error pattern of a color cathode ray tube apparatus has patterns represented in FIGS. 3E and 3F. In the recent years, however, as the panel of a color cathode ray tube is flatly formed, and complicated convergence and distortion correction mechanisms are added, a pattern in which the cross convergence error amount at each of the upper and lower end portions of the screen is smaller than that at each of the upper and lower intermediate portions of the screen, as shown in FIG. 3G, and a pattern in which the polarity of the cross convergence error at each of the upper and lower end portions of the screen is opposite to that at each of the upper and lower intermediate portions of the screen, as shown in FIG. 3H, are often formed.
In the conventional saturable reactor, since the correction amount is monotonously increased with respect to the vertical deflection current, although correction of the cross convergence error patterns as shown in FIGS. 3E and 3F is possible, it is difficult to correct the cross convergence error patterns as shown in FIGS. 3G and 3H. Hence, in the color cathode ray tube apparatus incorporating a conventional saturable reactor, a sufficient improvement in the image quality cannot be obtained.
Another screen distortion is a coma error which is generated since the deflection sensitivity for the center beam becomes relatively higher than that for a pair of side beams. More specifically, in the self convergence system in-line type color cathode ray tube apparatus, rasters 11B and 11R of the pair of side beams BB and BR can be set to coincide with each other throughout the entire area of the screen, as shown in FIG. 5, without requiring a correcting circuit means. However, due to the difference in deflection sensitivity between the center beam BG and the pair of side beams BB and BR, it is difficult to set a raster 11G of the center beam BG and the rasters 11B and 11R of the pair of side beams BB and RB to coincide with each other, and a coma error, i.e., horizontal and vertical direction coma errors HCR and VCR occur on each end of the horizontal axis (X axis) and each end of the vertical axis (Y axis), respectively, of the screen.
In the ordinary in-line type color cathode ray tube apparatus, this coma error can be corrected by disposing, to the electrode of the beam-emitting end portion of the electron gun assembly, a magnetic element called a field controller which has a function of relatively decreasing the deflection sensitivity for the pair of side beams to be lower than that for the center beam. When, however, a horizontal deflection frequency is changed to a high frequency, a convergence deviation is caused by the AC loss of the magnetic element. Therefore, many in-line type color cathode ray tube apparatuses correct the coma by the magnetic field of the deflection yoke itself without using a magnetic element. In this case, the coma error HCR can be corrected by the horizontal deflection coil itself as the deviation amount is small. However, it is difficult to correct the coma error VCR by the vertical deflection coil, as it has a large correction amount, and the coma error VCR remains uncorrected. Therefore, the coma error VCR is corrected by the following deflecting system. That is, auxiliary cores, obtained by winding coils respectively on a pair of U-shaped cores and connecting these coils to vertical deflection coils in series, are disposed at a side end portion (rear end portion) of the electron gun assembly of the deflection yoke to be vertically symmetric about the horizontal axis, and a pin cushion shape magnetic field is generated to correspond to the barrel vertical deflection magnetic field for vertical deflection. A means for controlling the operation of the auxiliary coils by diodes in order to efficiently correct the coma VCR throughout the entire area of the screen is shown in, e.g., Published Unexamined Japanese Patent Application No. 63-225462 (U.S. Pat. No. 4,818,919).
When the screen distortion is increased from the central portion of the screen toward the upper and lower end portions of the screen, as described above, it can be corrected to a certain degree. However, when the screen distortion at each of the upper and lower intermediate portions of the screen is larger than the screen distortion at each of the upper and lower end portions of the screen, sufficient correction cannot be performed.
Published Unexamined Japanese Patent Application Nos. 63-195935, 1-175150, and 1-183042 describe a means for forming a saturation control coil with two coils and controlling one coil by a diode. With this means, although cross convergence errors having patterns as shown in FIGS. 3E and 3F can be corrected, cross convergence errors having patterns as shown in FIGS. 3G and 3H cannot be corrected.